1. Field of the Invention
The present invention relates to control over a driving current which is supplied to an alternating-current motor including a rotor and a stator.
2. Description of the Related Art
A typical driving current supplied to a three phase alternating-current motor including a rotor and a stator is a current having three phases of iu, iv and iw. This three-phase driving current is controlled based on an output torque command from the motor. In a heretofore common technique for controlling such a motor, a current having respective phases (the respective phases of u, v and w) is converted into currents of d and q axis coordinate systems of an exciting current axis (a d axis) and a torque current axis (a q axis), and each converted axis current is controlled to match with an axis command value obtained from a torque command of the motor.
Such control assumes that a motor driving current basically conforms to a sine wave, and such control is directed at this fundamental wave component only. However, in reality, a magnetic flux generated in accordance with a motor driving current is distorted, or a higher harmonic component is generated in a motor driving current due to various situations such as characteristics at the time of inverter switching or the like.
Therefore, in order to perform further accurate control, control must be carried out taking a higher harmonic component into consideration.
As a control method for a higher harmonic current as a frequency component n times that of a fundamental wave component, there has been proposed preparation of a coordinate system by which a current can be processed as a direct current with respect to all current components as control targets and execution of control over a current transformed on the coordinate system and a command value (See, for example, Japanese Patent Application Laid-open No. 2002-223600).
However, in the above-described art, control over a higher harmonic current component is executed on a coordinate system for the higher harmonic current component. Therefore, after performing coordinate transformation from three phases of u, v and w (an αβ phase: a coordinate system with a stator fixed) into an axis coordinate system or the like, control or the like is performed, and an output obtained by this control is again subjected to reverse coordinate transformation. Therefore, because many coordinate transformation operations must be carried out, calculations are undesirably complicated.